Fuel cells are electrochemical cells that are being developed for motive and stationary electric power generation. One fuel cell design uses a solid polymer electrolyte (SPE) membrane or proton exchange membrane (PEM), to provide ion transport between the anode and cathode. Gaseous and liquid fuels capable of providing protons are used. Examples include hydrogen and methanol, with hydrogen being favored. Hydrogen is supplied to the fuel cell's anode. Oxygen (as air) is the cell oxidant and is supplied to the cell's cathode. The electrodes are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel to disperse over the surface of the membrane facing the fuel supply electrode. Each electrode has carried finely divided catalyst particles (for example, platinum particles), supported on carbon particles, to promote ionization of hydrogen at the anode and reduction of oxygen at the cathode. Protons flow from the anode through the ionically conductive polymer membrane to the cathode where they combine with oxygen to form water, which is discharged from the cell. Conductor plates carry away the electrons formed at the anode.
Currently, state of the art PEM fuel cells utilize a membrane made of perfluorinated ionomers such as DuPont's Nafion®. The ionomer carries pendant ionizable groups (e.g. sulfonate groups) for transport of protons through the membrane from the anode to the cathode.
A significant problem hindering the large scale implementation of fuel cell technology is the loss of performance during extended operation, the cycling of power demand during normal automotive vehicle operation as well as vehicle shut-down/start-up cycling. This invention is based on the recognition that a considerable part of the performance loss of PEM fuel cells is associated with the degradation of the oxygen reduction electrode catalyst. This degradation is probably caused by growth of platinum particles, dissolution of platinum particles, and corrosion of the carbon support material. The presence of sulfonate groups and water in the cell creates an acidic environment that contributes to these changes in the electrodes of each cell.
Carbon has been found to corrode severely at electrical potentials above 1.2V and the addition of platinum particles onto the surface of the carbon increases the corrosion rate considerably at potentials below 1.2V. These processes lead to a loss in active surface area of the platinum catalyst that leads to loss in oxygen electrode performance. However, cycling experiments have revealed that the loss of hydrogen adsorption area alone cannot explain the loss in oxygen reduction activity. Additional factors include interference from adsorbed OH species and a possible place-exchange of adsorbed OH species that can alter the electrocatalytic properties of the platinum catalyst towards oxygen reduction. Thus, the specific interaction of platinum with the catalyst support can have an enormous influence on the stability of performance of the Pt electrocatalyst.